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May 17th 14, 09:31 AM
posted to uk.radio.amateur,rec.radio.amateur.homebrew
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Output Impedance
On 17/05/2014 08:12, FranK Turner-Smith G3VKI wrote:
"gareth" wrote in message ... "gareth" wrote in message ... One thing that puzzled me for years was that the output impedance of an amplifying stage could be much higher than the voltage drop across the active element divided by the current through it. Now, clearly as current sources, this would be true, and it could be measured., by changing the loads. But, how could you calculate such a value up front? Interesting how the great blusterers display their ignorance, either by not entering the discussion by the device of side-stepping with abusive remarks*****, or trumpeting complete nonsense .... Consider the case of your homebrewed 1.5kW linear being set up with QRP for safety. Let's say 6W output on a 12V supply. Some claim that this means an impedance of 24 ohms that has to be matched to the 50 ohm antenna. So, having done your matching, you now crank up the output to 1.44kW by increasing the drive, and those some claimers will now say that the ouput impedance is now 0.1 ohm. Hang on there! By their argument, you will now be in danger of severely damaging your pride and joy by driving into what you say is a complete mismatch, set, not for 0.1 ohm but for 24 ohms. You would need to increase the supply volts to at least 200V to give headroom, unless you want a square wave output. Matthew 7:6 |
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